U.S. patent application number 10/987944 was filed with the patent office on 2006-06-01 for fast handover with reduced service interruption for high speed data channels in a wireless system.
This patent application is currently assigned to Lucent Technologies. Invention is credited to Patrick Georges Charriere, Fang-Chen Cheng, Philip Charles Sapiano.
Application Number | 20060116118 10/987944 |
Document ID | / |
Family ID | 35530758 |
Filed Date | 2006-06-01 |
United States Patent
Application |
20060116118 |
Kind Code |
A1 |
Charriere; Patrick Georges ;
et al. |
June 1, 2006 |
Fast handover with reduced service interruption for high speed data
channels in a wireless system
Abstract
In one aspect of the instant invention, a method is provided for
controlling a communications system that includes a mobile device,
a first and second base station and a radio network controller. The
method comprises establishing the first base station as a primary
base station that communicates high speed data to the mobile
device. A parameter associated with the first and a second base
stations, such as signal strength or quality, is monitored, and a
level one type signal is sent to the first and second base stations
indicating that the second base station is the primary base station
in response to the monitored parameter associated with the second
base station exceeding the monitored parameter associated with the
first base station.
Inventors: |
Charriere; Patrick Georges;
(Oxfordshire, GB) ; Cheng; Fang-Chen; (Randolph,
NJ) ; Sapiano; Philip Charles; (Wiltshire,
GB) |
Correspondence
Address: |
WILLIAMS, MORGAN & AMERSON
10333 RICHMOND, SUITE 1100
HOUSTON
TX
77042
US
|
Assignee: |
Lucent Technologies
|
Family ID: |
35530758 |
Appl. No.: |
10/987944 |
Filed: |
November 12, 2004 |
Current U.S.
Class: |
455/423 ;
455/453 |
Current CPC
Class: |
H04W 36/30 20130101 |
Class at
Publication: |
455/423 ;
455/453 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
1. A method for controlling a communications system, comprising:
establishing a first base station as a primary base station;
monitoring at least one parameter associated with the first base
station and a second base station; and sending a level one type
signal to the first and second base stations indicating that the
second base station is the primary base station in response to the
monitored parameters.
2. A method, as set forth in claim 1, further comprising reserving
resources in the second base station, wherein the reserved
resources are associated with communicating with the second base
station.
3. A method, as set forth in claim 2, wherein reserving resources
in the second base station further comprises reserving resources in
the second base station, wherein the reserved resources are
associated with communicating with the second base station using a
level one type signal.
4. A method, as set forth in claim 3, wherein reserving resources
in the second base station further comprises reserving resources in
the second base station, wherein the reserved resources are
associated with communicating with the second base station using
feedback information bits.
5. A method, as set forth in claim 1, wherein monitoring at least
one parameter associated with the first base station and the second
base station further comprises monitoring the parameter of a
communications channel associated with the first and second base
stations, wherein the parameter is quality.
6. A method, as set forth in claim 5, wherein sending the level one
signal to the first and second base stations indicating that the
second base station is the primary base station in response to the
monitored parameters further comprises sending the level one signal
to the first and second base stations indicating that the second
base station is the primary base station in response to the quality
of the communications channel associated with the second base
station exceeding the quality of the communications channel
associated with the first base station.
7. A method, as set forth in claim 1, wherein sending the level one
signal to the first and second base stations indicating that the
second base station is the primary base station in response to the
monitored parameters further comprises sending feedback information
bits to the first and second base stations indicating that the
second base station is the primary base station in response to the
monitored parameters.
8. A method for controlling a communications system including a
first and second base station and a radio network controller,
comprising: establishing the first base station as a primary base
station; receiving a level one type signal at the first and second
base stations indicating that the second base station is the
primary base station; and transmitting a signal from each of the
first and second base stations to the radio network controller
indicating that the second base station is the primary base
station; and directing data from the radio network controller to
the second base station.
9. A method, as set forth in claim 1, further comprising reserving
resources in the second base station, wherein the reserved
resources are associated with communicating between the second base
station and a mobile device.
10. A method, as set forth in claim 9, wherein reserving resources
in the second base station further comprises reserving resources in
the second base station, wherein the reserved resources are
associated with communicating between the second base station and
the mobile device using a level one type signal.
11. A method, as set forth in claim 10, wherein reserving resources
in the second base station further comprises reserving resources in
the second base station, wherein the reserved resources are
associated with communicating between the second base station and
the mobile device using feedback information bits.
12. A method, as set forth in claim 8, wherein receiving the level
one type signal at the first and second base stations indicating
that the second base station is the primary base station further
comprises receiving feedback information bits at the first and
second base stations indicating that the second base station is the
primary base station.
13. A method, as set forth in claim 8, wherein transmitting the
signal from each of the first and second base stations to the radio
network controller indicating that the second base station is the
primary base station further comprises sending a control frame.
14. A method, as set forth in claim 8, wherein transmitting the
signal from each of the first and second base stations to the radio
network controller indicating that the second base station is the
primary base station further comprises sending a frame protocol
control frame.
15. A method, as set forth in claim 8, wherein transmitting the
signal from each of the first and second base stations to the radio
network controller indicating that the second base station is the
primary base station further comprises sending an Iub/Iur frame
protocol control frame.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to telecommunications, and,
more particularly, to wireless communications.
[0003] 2. Description of the Related Art
[0004] In the field of wireless telecommunications, such as
cellular telephony, a system typically includes a plurality of base
stations distributed within an area to be serviced by the system.
Various users within the area, fixed or mobile, may then access the
system and, thus, other interconnected telecommunications systems,
via one or more of the base stations. Typically, a mobile device
maintains communications with the system as the mobile device
passes through an area by communicating with one and then another
base station, as the user moves. The mobile device may communicate
with the closest base station, the base station with the strongest
signal, the base station with a capacity sufficient to accept
communications, etc.
[0005] Historically, the mobile device has been used for voice
communications where the delivery of information is time critical.
That is, if even relatively short segments of a conversation are
delayed or lost, the meaning and understanding of the parties to
the conversation may be substantially impaired. During the period
when the mobile device is discontinuing communications with a first
base station and beginning communications with a second base
station, there is a distinct possibility that communications will
be at least temporarily interrupted or delayed. Thus, for voice
communications, a process known as soft hand off (SHO) was
developed in the CDMA and UMTS systems to have multiple connections
in the region of overlapped coverage in order to substantially
enhance the likelihood that the conversation will continue unabated
even during these transition periods.
[0006] Recently, the operation of mobile devices has been extended
to the field of high speed data, such as might be employed when
accessing the Internet or the World Wide Web. The exchange of high
speed data, unlike voice communications, has historically not been
time critical. That is, the transmission of data may be temporarily
interrupted or delayed without affecting a receiver's ability to
"understand" the data. Thus, temporary delays or interruptions
during the transition period from one base station to another have
been acceptable.
[0007] However, the use of high speed data connections has expanded
to operations that are more time critical. For example, Voice over
Internet Protcol (VoIP) is a process that involves digitizing voice
signals, organizing the digitized voice signals into packets, and
transmitting the packets over a high speed digital connection. A
receiving party reassembles the packets and plays the packets to
produce an audio communication. Thus, voice communications can be
accomplished over a high speed data connection. If this process can
be accomplished in real time, then a conversation may occur across
the high speed digital connection. Where the high speed digital
connection is being used for voice communications, then the
transition periods become significant so as to avoid delaying or
interrupting the conversation.
[0008] Typically, because data is not time sensitive, high speed
data channels have not implemented handover techniques to prevent
delays during the handoff process. Rather, the process of handover
has commonly been implemented using Level three (L-3) signaling,
which is relatively slow, further exacerbating the problem of
delays in the handoff process.
[0009] The present invention is directed to overcoming, or at least
reducing, the effects of one or more of the problems set forth
above.
SUMMARY OF THE INVENTION
[0010] In one aspect of the instant invention, a method is provided
for controlling a communications system. The method comprises
establishing a first base station as a primary base station. At
least one parameter associated with the first base station and a
second base station are monitored, and a level one type signal is
sent to the first and second base stations indicating that the
second base station is the primary base station in response to the
monitored parameters.
[0011] In another aspect of the instant invention, a method is
provided for controlling a communications system that includes a
first and second base station and a radio network controller. The
method comprises establishing the first base station as a primary
base station. A level one type signal is received at the first and
second base stations indicating that the second base station is the
primary base station, and a signal is transmitted from each of the
first and second base stations to the radio network controller
indicating that the second base station is the primary base
station. Thereafter, data from the radio network controller is
directed to the second base station.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention may be understood by reference to the
following description taken in conjunction with the accompanying
drawings, in which like reference numerals identify like elements,
and in which:
[0013] FIG. 1A is a block diagram of a communications system, in
accordance with one embodiment of the present invention;
[0014] FIG. 1B is a stylistic representation of a region in which
the communications system of FIG. 1A may be employed;
[0015] FIG. 2 depicts a block diagram of one embodiment of a Base
station and a mobile device used in the communications system of
FIG. 1;
[0016] FIG. 3 is a flow diagram illustrating the interoperation of
the various components of the communications system of FIGS. 1 and
2;
[0017] FIG. 4 is a flow diagram illustrating one embodiment of a
control strategy employed in the communications system of FIGS. 1-3
for handling handover between a first and second base station.
[0018] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof have been shown
by way of example in the drawings and are herein described in
detail. It should be understood, however, that the description
herein of specific embodiments is not intended to limit the
invention to the particular forms disclosed, but on the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0019] Illustrative embodiments of the invention are described
below. In the interest of clarity, not all features of an actual
implementation are described in this specification. It will of
course be appreciated that in the development of any such actual
embodiment, numerous implementation-specific decisions must be made
to achieve the developers' specific goals, such as compliance with
system-related and business-related constraints, which will vary
from one implementation to another. Moreover, it will be
appreciated that such a development effort might be complex and
time-consuming, but would nevertheless be a routine undertaking for
those of ordinary skill in the art having the benefit of this
disclosure.
[0020] Turning now to the drawings, and specifically referring to
FIG. 1A, a communications system 100 is illustrated, in accordance
with one embodiment of the present invention. For illustrative
purposes, the communications system 100 of FIG. 1A is a Universal
Mobile Telephone System (UMTS), although it should be understood
that the present invention may be applicable to other systems that
support data and/or voice communication. The communications system
100 allows one or more mobile devices 120 to communicate with a
data network 125, such as the Internet, and/or a public telephone
system (PSTN) 160 through one or more base stations 130. The mobile
device 120 may take the form of any of a variety of devices,
including cellular phones, personal digital assistants (PDAs),
laptop computers, digital pagers, wireless cards, and any other
device capable of accessing the data network 125 and/or the PSTN
160 through the base station 130.
[0021] In one embodiment, a plurality of the base stations 130 may
be coupled to a Radio Network Controller (RNC) 138 by one or more
connections 139, such as T1/EI lines or circuits, ATM virtual
circuits, cables, optical digital subscriber lines (DSLs), and the
like. Although one RNC 138 is illustrated, those skilled in the art
will appreciate that a plurality of RNCs 138 may be utilized to
interface with a large number of base stations 130. Generally, the
RNC 138 operates to control and coordinate the base stations 130 to
which it is connected. The RNC 138 of FIG. 1 generally provides
replication, communications, runtime, and system management
services, and, as discussed below in more detail below, may be
involved in coordinating the transition of a mobile device 120
during transitions between the base stations 130.
[0022] As is illustrated in FIG. 1B, a region 170 to be serviced by
the system 100 is separated into a plurality of regions or cells,
each being associated with a separate base station 130. Typically,
each cell has a plurality of adjacent neighboring cells. For
example, the cell 175 has six neighboring cells 176-181 such that a
mobile device 120 entering the cell 175 may travel from one of the
neighboring cells 176-181. Thus, as the mobile device 120 enters
the cell 175 from any of the neighboring cells 176-181, the mobile
device may need to transition from communicating with the cell 175
to communicating with the neighboring cell 176-181 that it is
entering.
[0023] Returning to FIG. 1A, the RNC 138 is also coupled to a Core
Network (CN) 165 via a connection 145, which may take on any of a
variety of forms, such as T1/EI lines or circuits, ATM virtual
circuits, cables, optical digital subscriber lines (DSLs), and the
like. Generally the CN 165 operates as an interface to the data
network 125 and/or to the public telephone system (PSTN) 160. The
CN 165 performs a variety of functions and operations, such as user
authentication, however, a detailed description of the structure
and operation of the CN 165 is not necessary to an understanding
and appreciation of the instant invention. Accordingly, to avoid
unnecessarily obfuscating the instant invention, further details of
the CN 165 are not presented herein.
[0024] Thus, those skilled in the art will appreciate that the
communications system 100 enables the mobile devices 120 to
communicate with the data network 125 and/or the PSTN 160. It
should be understood, however, that the configuration of the
communications system 100 of FIG. 1A is exemplary in nature, and
that fewer or additional components may be employed in other
embodiments of the communications system 100 without departing from
the spirit and scope of the instant invention.
[0025] Unless specifically stated otherwise, or as is apparent from
the discussion, terms such as "processing" or "computing" or
"calculating" or "determining" or "displaying" or the like, refer
to the action and processes of a computer system, or similar
electronic computing device, that manipulates and transforms data
represented as physical, electronic quantities within the computer
system's registers and memories into other data similarly
represented as physical quantities within the computer system's
memories or registers or other such information storage,
transmission or display devices.
[0026] Referring now to FIG. 2, a block diagram of one embodiment
of a functional structure associated with an exemplary base station
130 and mobile device 120 is shown. The base station 130 includes
an interface unit 200, a controller 210, an antenna 215 and a
plurality of channels: such as a shared channel 220, a data channel
230, and a control channel 240. The interface unit 200, in the
illustrated embodiment, controls the flow of information between
the base station 130 and the RNC 138 (see FIG. 1A). The controller
210 generally operates to control both the transmission and
reception of data and control signals over the antenna 215 and the
plurality of channels 220, 230, 240 and to communicate at least
portions of the received information to the RNC 138 via the
interface unit 200.
[0027] The mobile device 120 shares certain functional attributes
with the base station 130. For example, the mobile device 120
includes a controller 250, an antenna 255 and a plurality of
channels: such as a shared channel 260, a data channel 270, and a
control channel 280. The controller 250 generally operates to
control both the transmission and reception of data and control
signals over the antenna 255 and the plurality of channels 260,
270, 280.
[0028] Normally, the channels 260, 270, 280 in the mobile device
120 communicate with the corresponding channels 220, 230, 240 in
the base station 130. Under the operation of the controllers 210,
250, the channels 220, 260; 230, 270; 240, 280 are used to effect a
controlled scheduling of communications from the mobile device 120
to the base station 130.
[0029] Turning now to FIG. 3, a flow diagram illustrating the
interoperation of the various components of the system 100 is
shown. In the flow diagram of FIG. 3, it is assumed that a high
speed data transmission is underway with respect to the mobile
device 120 such that the mobile device 120 is communicating with
base station A, but will be transitioning to base station B.
Initially, the mobile device 120 is within the cell associated with
base station A and is approaching or entering the cell associated
with the base station B.
[0030] FIG. 3 stylistically represents a handover procedure for a
high speed data channel, generally depicting a messaging process
that may be used to switch over the high speed data channel from a
serving cell to a target cell. Generally, the actual switchover
begins when the RNC 138 sends a Radio Link Reconfiguration Commit
messages to the serving Base station 130 to stop the scheduled
transmission at a defined "activation time." The mobile device 120
begins "listening" to scheduling information from the target cell
at the activation time after sending the "Physical Channel
Reconfiguration Complete" messages. A significant factor in
reducing VoIP service interruption is the setting of the
"activation time" for the switchover from the serving cell to the
target cell. Historically, the "activation time" is set at the very
early stage of Radio Link Reconfiguration Commit messages and is
executed at the time when the mobile device 120 sends the "Physical
Channel Reconfiguration Complete." In some prior art systems, the
setting time and the execution time may be separated by a
significant amount of time (e.g., from hundreds of milliseconds to
seconds). The "activation time" is determined based on the process
time of the signaling messages (e.g., Iur, Iub and UU). During this
interval, a variety of factors could negatively impact the radio
channel conditions. For example, the mobile device 120 may move
away from the serving cell and the radio link quality could
deteriorate significantly such that data could no longer be
delivered. It could also happen that mobile device 120 may be
unable to decode the scheduling information and missed the data.
Thus, VoIP service to the mobile device 120 may be interrupted for
a significant amount of time. In one embodiment of the instant
invention, the activation time for the switchover process is
carefully selected based on current conditions of the radio link
between the mobile device 120 and the serving and target base
stations.
[0031] In a first embodiment of the instant invention, as shown in
FIG. 3, the serving RNC 138-1 determines that there is a need to
switchover from high speed communications between the mobile device
120 and a source base station 130-1 to high speed communications
between the mobile device 120 and a target base station 130-2. The
serving RNC 138-1 prepares an RNSAP (Radio Link Reconfiguration
Prepare) message, which is transmitted to a drift RNC 138-2 at
300.
[0032] In the illustrated embodiment, the source and target cells
are controlled by different base stations 130-1, 130-2. The drift
RNC 138-2 requests the source base station 130-1 to perform a
synchronized radio link reconfiguration using an NBAP (Base station
130 Application Part) message "Radio Link Reconfiguration Prepare"
at 302. The source base station 130-1 returns an NBAP message
"Radio Link Reconfiguration Ready" at 304.
[0033] The drift RNC 138-2 requests the target base station 130-2
to perform a synchronized radio link reconfiguration using the NBAP
message "Radio Link Reconfiguration Prepare" at 306. The target
base station 130-2 returns the NBAP message "Radio Link
Reconfiguration Ready" at 308. The drift RNC 138-2 returns an RNSAP
message "Radio Link Reconfiguration Ready" to the serving RNC 138-2
at 310. The drift RNC 138-2 initiates set-up of a new Iub Data
Transport Bearers using ALCAP (Access Link Control Application
Protocol) protocol at 312. This request contains an AAL2 (ATM
Adaptation Layer type 2) Binding Identity to bind the Iub Data
Transport Bearer to the high speed data channel.
[0034] The serving RNC 138-1 initiates set-up of a new Iur Data
Transport bearer using ALCAP protocol at 314. This request contains
the AAL2 Binding Identity to bind the Iur Data Transport Bearer to
the high speed data channel. The high speed data channel transport
bearer to the target base station 130-2 is established. The serving
RNC 138-1 proceeds by transmitting the RNSAP message "Radio Link
Reconfiguration Commit" to the drift RNC 138-2 at 316. The serving
RNC 138-1 selected the activation time in the form of a CFN
(Connection Frame Number).
[0035] The drift RNC 138-2 transmits the NBAP message "Radio Link
Reconfiguration Commit" to the source base station 130-1 including
the activation time at 318. Similarly, the drift RNC 138-2 also
transmits the NBAP message "Radio Link Reconfiguration Commit" to
the target base station 130-2 including the activation time at 320.
At the indicated activation time, the source base station 130-1
stops transmitting and the target base station 130-2 starts
transmitting on the high speed data channel to the mobile device
120.
[0036] The serving RNC 138-1 also transmits an RRC (Radio Resource
Control) message "Physical Channel Reconfiguration to the mobile
device 120 at 322. At the indicated activation time the mobile
device 120 stops receiving high speed data in the source cell and
starts receiving high speed data in the target cell. The mobile
device 120 returns an RRC message "Physical Channel Reconfiguration
Complete" to the serving RNC 138-1 at 324. The drift RNC 138-2
initiates release of the old Iub Data Transport bearer using ALCAP
protocol at 326. Similarly, the serving RNC 138-1 initiates release
of the old Iur Data Transport bearer using ALCAP protocol at
328.
[0037] To reduce or minimize VoIP service interruption during
handover, the "activation time" needs to be properly selected. The
"activation time" is determined by the processing time of the
signaling procedure. If the switchover is set very late and the
radio channel condition of the serving cell has deteriorated, the
VoIP service will have an open window period of no service. If the
activation time is set too early and the radio channel condition of
the target cell relative to the specific user is not good, the VoIP
service will have an interruption too. The issue of the switchover
timing is the unawareness of the radio channel condition when RNC
138-1 sets the activation time. It requires L-3 signaling to
communicate between the mobile device 120 and RNC in order to
identify the best timing of switchover. Nevertheless, the L-3
signaling delay or latency is large enough to affect the
effectiveness of the switchover timing identification. Thus, the
criteria of minimizing the VoIP service interruption during the
handover are to estimate the required signaling processing time
correctly, to reduce the signaling time or latency, and to switch
over to the target cell at the right time.
[0038] Reducing the latency or time required for signaling during
switchover can improve the switchover process. The RNC 138 uses
feedback of the radio channel conditions, such as SIR (Signal to
Interference Ratio) from the base stations 130 in the active set or
mobile reported best cell measurement, to trigger the switchover
and to estimate a desired switchover time. However, the accuracy of
the estimation deteriorates as the prediction interval becomes
larger. In particular, radio channel conditions may changes
dramatically in hundreds of milliseconds to seconds. By reducing
the interval, the switchover process may be substantially
improved.
[0039] The base station 130 periodically reports SIR measurements
to the RNC 138. In one embodiment, the average SIR is calculated
over an 80 ms interval and reported to the RNC 138. The average SIR
measurement may be used as a reference of the radio channel
condition of each leg during the switchover. The average uplink SIR
measurements are a reciprocal of the long-term average of the CQI
reports for the downlink radio link. In one embodiment of the
instant invention, an algorithm used to calculate "activation time"
is as follows, { SIR Serving < Threshold Serving SIR Target >
Threshold target ##EQU1## where SIR.sub.Serving and SIR.sub.Target
are the SIR measurements of the serving cell and the target cell,
respectively, and Threshold.sub.Serving and Threshold.sub.target
are the thresholds of the serving cell and target cell,
respectively. The algorithm is designed based on link imbalance
during the switchover when the uplink inner loop power control
would speed up the migration of the radio link to the leg with good
channel condition. Thus, the activation time setting could be
improved by setting the threshold properly in accordance with the
signaling process delay or latency.
[0040] The general signaling procedures of switching the high speed
data channel from the serving cell to the target cell are performed
in series. Since Iub Radio Link Reconfiguration Commit message
requires no acknowledgement, the delay may be reduced by sending
Iub Radio Link Reconfiguration messages to the serving and target
cells as well as UU RRC Physical Layer Reconfiguration message to
the mobile device 120 at about the same time, or at least with some
overlap. This would remove significant delays and introduce minimum
process delay only.
[0041] With the concurrent Iub and UU signaling message to commit
the switchover of the radio links at the activation time, the
processing delay of the UU RRC Physical Channel Reconfiguration
message is another factor in the delay. The current performance
procedure for the Physical Channel Reconfiguration (see section
13.5.2 of TS25.331) requires the mobile device 120 to execute it
within 80 ms after receiving the UU signaling message and update
the L1 configuration at the beginning of the next TTI. The DCCH
RABs supported in the TS34.108 are 1.7 kbps, 3.4 kbps, and 13.6
kbps with TTI being 80, 40, and 10 ms, respectively. The most
common test procedure is the 3.4 kbps DCCH with 40 ms TTI. If the
3.4 kbps DCCH were used to carry the Physical Channel
Reconfiguration messages for the high speed data channel
switchover, the total delay would exceed 120 ms (80 ms
processing+40 ms TTI DCCH reception). Moreover, the 80 ms execution
delay is also considered by the low signaling RAB for communication
of the primitive between higher layer protocols and physical layers
at the mobile device 120. Using the 13.6 kbps RAB with 10 ms TTI
would reduce the execution time and processing time. It will reduce
the interruption of the VoIP service during switchover.
[0042] One significant issue of the high speed data channel
switchover is to synchronize the buffers at the base stations 130-1
and 130-2 since both Iub links for the serving cell and target cell
are established before the Radio Link Reconfiguration Commit
commands. To reduce the service interruption, the RNC 138 can send
the VoIP data to both of the base stations 130-1 and 130-2 after
the Iub data link of the target base station 130-2 is
established.
[0043] The target base station 130-2 will not schedule any
transmission before the "activation time" and has no idea of the
scheduled information at the source base station 130-1. Since other
channels (e.g., UL DPCH) are operating in the soft-handover mode,
the target base station 130-2 may decode the Ack/Nack and CQI
information before the "activation time" although that they intend
to send to the source base station 130-1. Due to the link imbalance
during the switchover, the target cell would decode the Ack/Nack
and CQI for the serving cell incorrectly with erasure in the
beginning and gradually move into correct decoding before the
"activation time". The target cell could decide to drop a VoIP
frame every T.sub.erasure interval if the Ack/Nack and CQI are
erased. The T.sub.erasure interval is a parameter to be optimized
for minimizing the buffer occupancy at the target cell. If the Ack
is decoded correctly at the target cell before the activation time,
the target cell could determine to drop the next VoIP packet. This
procedure is an uncoordinated VoIP counting for the high speed data
channel when the VoIP packet has been sent to both the serving and
target cells during the switchover before the activation time. This
will avoid any data gap for the sequence delivery of the VoIP
packet and minimize the buffer occupancy at the target cell. The
proposed algorithm is called "Pseudo-Synchronization" because its
behavior is similar to the buffer synchronization.
[0044] One cause of the signal processing delay or latency arises
from the use of Layer 3 (L-3) signaling procedures during the
handover process. As discussed above, to reduce quality degradation
for VOIP over a high speed data channel, it may be useful in some
applications to reduce the latency of the switchover during the
handover process. One method that may be employed to reduce latency
involves using Site Selection Diversity Transmit (SSDT) type Layer
1 (L-1) signaling to assist channel switchover during handover.
Additionally, plane signaling in the UMTS Terrestrial Random Access
Network (UTRAN) may also be used to assist the switchover of the
data transfer from the RNC 138 to the serving base station 130-1 to
the target base station 130-2.
[0045] Generally, in one embodiment of the instant invention, L-1
signaling is used to identify the primary serving cell during the
handover after measurements made by the mobile device 120 indicate
the best cell is not the current serving cell. Based on these
measurements, the mobile device 120 indicates its new primary
serving or target cell. Once the cells receive the indication of
the new primary serving cell from the mobile device 120, all cells
send the primary/non-primary cell indications to the RNC 138. The
RNC 138 responds by switching the user plane traffic to the Iub/Iur
transport of the new primary or target cell. The L-3 signaling
messages are subsequently used to complete the handover procedure.
The proposed solution will reduce the delay caused by L-3
signaling.
[0046] Turning to FIG. 4, a stylized representation of one
embodiment of a handover control strategy 400 is illustrated. The
process begins at block 405 with the mobile device 120 monitoring
certain parameters of at least a portion of the base stations in
its active set to determine the quality of communications. While a
communications session is only established with the primary serving
cell, the mobile device 120 nonetheless monitors one or more
channels of the other base stations in its active set to determine
the quality of communications that would be available should a
handover occur. At block 410, the mobile device 120 makes a request
of at least a portion of the base stations 130 in its active set
that resources needed to conduct high speed communications be
reserved.
[0047] At block 415, the mobile device 120, using the monitored
parameters of each of the base stations 130 determines whether a
handover is warranted. If not, control returns to block 405 where
the process repeats. On the other hand, if the monitored parameters
indicate that a handover should be performed, then control
transfers to block 420 where the mobile device 120 communicates its
desire for a handover to occur to a select one of the monitored
base stations 130. The mobile device 120 provides this indication
to at least the primary serving cell and the target cell. At block
425, each of the base stations 130 communicates the handover to the
RNC 138. Thereafter, the RNC 138 effects the handover in a manner
as described above, ceasing the delivery of high speed data to the
primary serving cell and beginning the delivery of high speed data
to the target cell at a preselected time.
[0048] During a soft handover, the RNC 138 sends the following
information to the mobile device 120: the cells that the mobile
device 120 should be monitoring; the radio channel information
(HS-SCCH code number, H-RNTI , etc.) for any new cells to be
monitored; and the expected MAC-hs TSN reset during cell
change.
[0049] The mobile device 120 will monitor a high speed Shared
Control Channel (HS-SCCH) in all cells that it is commanded to
monitor in the active set (in some embodiments of the invention
this may be the whole active set) during the soft handover and
store information regarding each. The stored information will
enable the mobile device 120 to react quickly when radio channel
condition changes. The number of cells that a mobile device 120 may
be instructed to monitor may be determined as a function of the
type or capability of the mobile device 120. Additionally, the RNC
138 can periodically re-assign which cells in the active set that
the mobile device 120 should monitor.
[0050] Generally, all cells considered as candidate cells for fast
cell selection (FCS) are reserved with high speed data shared
channel (HS-DSCH) resources, which will enable the mobile device
120 to report FeedBack Information (FBI) bits to the base station
130 to identify the primary base station, and to send high speed
data from the primary base station as commanded by the RNC 138.
[0051] As discussed above, resources are reserved to get ready for
the high speed data serving link switchover. In one embodiment of
the instant invention, Iub, Iur, and all network node process
resources are reserved. In particular, the Iub and Iur RL
Reconfiguration Prepare and ALCAP procedures will be set at the
same time when DPCH is in soft handover. The mobile device 120
performs a best cell selection measurement and the measurement
results are reported back to the RNC 138 through L-1 signaling
(instead of RRC signaling, as was done in prior systems). The
results of the best cell selection by the mobile device 120
triggers the primary cell selection through the signal strength
measurements via FBI bits to report to the base stations for
primary base station indication. Multiple Iub/Iur links are set up
but only the primary cell Iub link is active at a given time. Thus,
there is no significant increase in backhaul traffic volume.
Alternatively, the RNC 138 may set up the high speed data channel
resources and transport resources in all cells, and the base
station 130 will remain silent on the high speed control channel
until such a time that the RNC 138 forwards some data to it.
[0052] The FBI bits in the up-link data control channel (UL DPCCH)
slot format 2, 3, 4, and 5 may be used to indicate the switchover
of the serving cell. The primary cell indication is used for the
fast cell selection instead of dynamic serving cell selection (ping
pong) during the soft handover of the SSDT feature.
[0053] In one embodiment of the instant invention, the Cell ID of
the FBI bits sent by the mobile device 120 is accumulated in
interval N slots (depending on Long, Medium, or Short ID code and
FBI bits format) in sliding window fashion at the base station 130.
If N consecutive primary ID codes are received and identified at
the target cell, the target cell shall consider itself as the
primary cell and will be switched over to the primary serving cell
in T ms. The accumulation of Cell-ID codes would also improve the
reliability of the FBI bit decoding since the FBI bits are 15 or 30
bits per radio frame without channel coding protection. The
interval (N slots) is a configurable parameter and would be
optimized on the field.
[0054] Alternatively, the Base station 130 can just forward the
decoded Cell-ID for the primary cell by the user plane frame up to
the RNC 138 and the RNC 138 can switch the user plane data to the
new cell.
[0055] The FBI bit generation for the L-1 signaling may be
triggered by the same criteria as best cell selection. The
measurement occurs within the mobile device 120 and does not need
to be reported back to the RNC 138. Alternatively, the mobile
device 120 will send the Cell-ID FBI bits for the L-1 signaling T
ms ahead of the actual switchover to allow the RNC 138 to switch
the Iub/Iur link from the current cell to the target cell. The time
advance T is a configurable parameter.
[0056] In an alternative embodiment of the instant invention, a
particular Cell ID may be regarded as no cell change to the mobile
device 120 and base station 130, and the base station 130 would not
forward this information up to the RNC 138. In this way the
signaling load could be reduced.
[0057] Turning now to a discussion of user plane signaling for
backhaul (Iub/Iur Frame Protocol for the HSDPA primary cell
indication, the fastest way to have the RNC 138 switch the high
speed data from the serving cell to the target cell is the Iub/Iur
Frame Protocol.
[0058] A proposal currently exists to change the Third Generation
Partnership Project (3GPP) standard Iub/Iur Frame Protocol to have
the header contain an indication of the primary cell/non-primary
cell during the handover when the mobile device 120 stored multiple
high speed data configurations and UMTS Terrestrial Random Access
Network (UTRAN) already set up the Iub/Iur link for the target
cells in the active set. Under the proposed change, once the base
station 130 receives the Cell-ID through FBI bits to indicate the
switchover of the serving cells, all base stations 130 will send UL
Iub/Iur Frame protocol control frame to indicate the status of
primary or non-primary serving cell for each cell. The RNC 138 will
start sending the data to the new primary cell after the reception
of the UL Iub/Iur frame protocol with indication of the serving
cell switchover.
[0059] The Iub/Iur protocol could also be enhanced to include the
MAC-d flow pointer being served by the current serving cell to
feedback to the RNC 138. This will allow the RNC 138 to sync-up the
MAC-d served by the current base station 130.
[0060] Note that during the period just before the mobile device
120 handover, the RNC 138 may hold the DL MAC-d data flow in its
buffer to give a chance to the old primary leg to flush its MAC-HS
priority queue before the switchover. The configured advanced
switchover time (T ms) is designed to allow the current primary
cell to serve out all the MAC-d data in its buffer. This would
reduce data loss during the handover. The mobile device 120 will
deduce that the cell change has occurred through the decoded
transmission of the HS-DSCH Radio Network Temporal Identifier
(H-RNTI) on the high speed control channel of the new cell. The
mobile device 120 will then determine whether to perform a MAC-hs
Transmission Sequence Number (TSN) reset and start decoding from
the new cell.
[0061] The L-3 signaling will follow the actual handover to
complete the high speed data channel handover procedure. The
current RL Reconfiguration Commit and RRC Physical Channel
Reconfiguration messages will follow up after the L-1 signaling.
The "activation time" would set to "now" if the L-1 signaling were
used for the indication of the switchover.
[0062] Those skilled in the art will appreciate that the FCS fast
handover for the VOIP over high speed data channel described herein
uses L-1 signaling to quickly indicate the switchover of the
serving cell. This will reduce service interruption during the hard
handover of the high speed service.
[0063] Those skilled in the art will appreciate that the various
system layers, routines, or modules illustrated in the various
embodiments herein may be executable control units (such as the
controllers 210, 250 (see FIG. 2)). The controllers 210, 250 may
include a microprocessor, a microcontroller, a digital signal
processor, a processor card (including one or more microprocessors
or controllers), or other control or computing devices. The storage
devices referred to in this discussion may include one or more
machine-readable storage media for storing data and instructions.
The storage media may include different forms of memory including
semiconductor memory devices such as dynamic or static random
access memories (DRAMs or SRAMs), erasable and programmable
read-only memories (EPROMs), electrically erasable and programmable
read-only memories (EEPROMs) and flash memories; magnetic disks
such as fixed, floppy, removable disks; other magnetic media
including tape; and optical media such as compact disks (CDs) or
digital video disks (DVDs). Instructions that make up the various
software layers, routines, or modules in the various systems may be
stored in respective storage devices. The instructions when
executed by the controllers 210, 250 cause the corresponding system
to perform programmed acts.
[0064] The particular embodiments disclosed above are illustrative
only, as the invention may be modified and practiced in different
but equivalent manners apparent to those skilled in the art having
the benefit of the teachings herein. Furthermore, no limitations
are intended to the details of construction or design herein shown,
other than as described in the claims below. Consequently, the
method, system and portions thereof and of the described method and
system may be implemented in different locations, such as the
wireless unit, the base station, a base station controller and/or
mobile switching center. Moreover, processing circuitry required to
implement and use the described system may be implemented in
application specific integrated circuits, software-driven
processing circuitry, firmware, programmable logic devices,
hardware, discrete components or arrangements of the above
components as would be understood by one of ordinary skill in the
art with the benefit of this disclosure. It is therefore evident
that the particular embodiments disclosed above may be altered or
modified and all such variations are considered within the scope
and spirit of the invention. Accordingly, the protection sought
herein is as set forth in the claims below.
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